The present invention relates, in general, to power amplifiers and, more particularly, to an apparatus, method and system for high efficiency, wideband linear power amplification in wireless applications, such as broadband CDMA and 3G cellular systems.
Power amplifiers for wireless transmission applications, such as radio frequency (xe2x80x9cRFxe2x80x9d) power amplifiers, are utilized in a wide variety of communications and other electronic applications. Ideally, the input-output transfer function of a power amplifier should be linear, with a perfect replica of the input signal, increased in amplitude, appearing at the output of the power amplifier.
In addition, for greater efficiency, various RF systems, such as cellular systems, attempt to run power amplifiers at or near their saturation levels, in which the actual output power of the amplifier is just below its maximum rated power output level. This power output level is generally related to the supply voltage (or supply power) to the power amplifier, such that a greater supply voltage will produce a correspondingly greater output power from the amplifier; for higher power input signals, a correspondingly greater actual power output is required to maintain the amplifier at or near saturation. In various prior art amplifiers, however, the supply voltage to the power amplifier is fixed. Given a typical usage situation in which actual power output from the amplifier may vary by a range of several orders of magnitude, use of a fixed supply voltage is highly inefficient, as output power is often an order of magnitude below its maximum, and the power amplifier is not maintained at or near its saturation levels.
Various techniques have evolved to vary the supply voltage to maintain the power amplifier at or near saturation. One such technique is power supply modulation (xe2x80x9cPSMxe2x80x9d) which varies, or modulates, the supply voltage to the power amplifier, in order to maintain the power amplifier at or near saturation while the input signal varies over time. For PSM, the supply voltage of the amplifier tracks the input signal variations, typically utilizing a signal detector in conjunction with a tracking power supply. In the prior art, however, the various PSM techniques have generally been limited to narrowband applications, or have poor efficiency characteristics.
For example, one prior art PSM technique, known as Envelope Elimination Restoration (xe2x80x9cEERxe2x80x9d), utilizes a limiter to provide an essentially constant drive level to the power amplifier to maintain the amplifier in a hard saturation state and increase efficiency. Use of the limiter, however, greatly expands the bandwidth of the RF signal input to the amplifier, and requires very accurate tracking of the input signal envelope, with a power supply switching frequency approximately ten times greater than the bandwidth of the RF input signal. As these switching frequencies increase, the transistors within the tracking power supply become less efficient, resulting in excessive power losses. The resulting bandwidth expansion of the limiter also requires the bandwidth capability of the amplifier to be significantly greater than the input signal bandwidth, limiting the EER configuration to narrow bandwidth applications, such as amplitude modulation (xe2x80x9cAMxe2x80x9d) RF broadcasts.
Another prior art PSM technique, known as Envelope Tracking (xe2x80x9cETxe2x80x9d), does not utilize the limiter of EER, and consequently may be suitable for higher bandwidth applications. Use of envelope tracking, however, introduces significant non-linearities in the output signal of the power amplifier, such as gain distortions, phase distortions, and other voltage parasitics. More particularly, while power amplifiers comprised of LDMOS (laterally diffused MOSFET) circuitry have good linearity with respect to input power, such LDMOS and other types of power amplifiers have large variations in gain and phase as a function of supply voltage. When PSM techniques are utilized for amplification efficiency, these non-linearities cause intermodulation distortion (xe2x80x9cIMDxe2x80x9d) in multicarrier frequency division multiple access (xe2x80x9cFDMAxe2x80x9d) or time division multiple access (xe2x80x9cTDMAxe2x80x9d) systems, and spectral growth in code division multiple access (xe2x80x9cCDMAxe2x80x9d) systems. These various distortions also degrade output signal quality and may have other detrimental effects, such as decreased data throughput.
As a consequence, a need remains for an apparatus, method and system to provide high efficiency power amplification in broadband (or wideband) applications, such as 3G and other wideband cellular or RF applications. Such an apparatus, method and system should provide linear power amplification, minimizing any phase, gain, and other distortions. In addition, such an apparatus, method and system should be cost-effective and capable of implementation in existing RF transmission systems, such as existing cellular base stations.
An apparatus, method and system are provided for power amplification of an input signal to produce a substantially linear amplified output signal, for broadband wireless applications such as 3G cellular and broadband CDMA systems, without creating significant intermodulation distortion or spectral growth.
The preferred system 100 embodiment includes an envelope detector 110, a tracking power supply 120, an input signal conditioner 150, and a power amplifier 170. The envelope detector 110 is utilized to determine an envelope detector voltage from the input signal, and the tracking power supply 120 is utilized to determine a supply voltage from the envelope detector voltage, preferably as a substantially quantized version of the envelope detector voltage. The supply voltage is utilized to maintain the power amplifier 170 at or near its saturation level. An input signal conditioner 150 is utilized to predistort or condition the input signal, to form a conditioned input signal, such that when the conditioned input signal is amplified by the power amplifier 170 using the supply voltage, the amplified output signal is a substantially linear, amplified replica of the input signal.
In response to the supply voltage, the input signal conditioner 150 determines a corresponding phase adjustment and a corresponding gain adjustment. In the preferred embodiment, the corresponding phase adjustment is selected from a plurality of predetermined phase adjustments in response to the supply voltage, and the corresponding gain adjustment is selected from a plurality of predetermined gain adjustments, also in response to the supply voltage. The plurality of predetermined phase adjustments and the plurality of predetermined gain adjustments are determined separately and independently, preferably from respective piecewise linear mappings of a phase response and a gain response of the power amplifier to a range of variation of the supply voltage. The input signal conditioner then modifies the input signal using the corresponding phase adjustment and the corresponding gain adjustment to produce the conditioned input signal.
The preferred system 100 embodiment also includes a first delay circuit 160, to synchronize the input signal with the supply voltage, and a second delay circuit 165 to synchronize the supply voltage with the conditioned input signal.
The apparatus, method and system embodiments of the present invention provide for high efficiency power amplification in broadband (or wideband) applications, such as 3G and other wideband cellular or RF applications, without significant bandwidth limitations, without intermodulation distortion, and without spectral growth. The various embodiments of the present invention effectively provide linear power amplification, minimizing any phase, gain, and other distortions. In addition, the apparatus, method and system of the present invention are cost-effective and capable of implementation in existing RF transmission systems, such as existing cellular base stations.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.